A 68 yo woman with known COPD on home oxygen, presents a little drowsy. Her initial CO2 on venous gases was 84mmHg. She is placed on a short course of CPAP and is soon sitting out of bed eating sandwiches. The medical registrar looks at the venous gases and doesn’t want to take the patient to the ward because of the CO2. What do you think?

pH 7.25pCO2 74 mmHgpO2 17 mmHgHCO3- 33 mmol/L

Solving gases like this is relatively simple.

THE FIRST THING TO SAY IS: This patient should not be going to ICU anyway, as she is on home oxygen. The only question to ask is whether she OKAY to leave the ED and go to the ward?

I say yes! My suspicion in this patient, is that she normally is compensated for a high CO2. Lets see if that is correct....

Start as always with the acid-base balance. This is part of the OWN the ABG method. We'll be covering ABGs in more detail in a few weeks. (Check your curriculum!)

Returning to the ABG and looking at the numbers we can see the patient has:

an ACIDAEMIA

HYPERCARBIA (respiratory acidosis)

HIGH BICARBONATE (metabolic alkalosis).

On the assumption that the primary disturbance always occurs in the direction of the pH derangement we can state that this patient has a PRIMARY RESPIRATORY ACIDOSIS, with a COMPENSATORY METABOLIC ALKALOSIS.

Therefore looking at this patient's CO2 our rise is approximately 35mmHg above 40mmHg.

If the patient had an ACUTE respiratory acidosis this would translate to a total bicarbonate of 27.5mmol/L

For a chronic respiratory acidosis this would equate to a bicarbonate of 38mmol/L

So our patient has overcompensated for an acute picture, and undercompensated for a chronic one. Therefore this patient must have an element of acute on chronic respiratory acidosis. After a short course of NIPPV we have probably returned this patient quite close to her baseline and she should be fit for the ward, especially given the improvement in her clinical state.

Now, the good news is in the new exam you don't have to write all that. The bad news is you still have to think it!